The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

Guth recalled a lecture by Bob Dicke that he had attended the previous year, one of a series that Dicke and Peebles had been delivering on a topic they called the “flatness problem.” They would explain to their audiences that the fate of the universe depended on how much matter was in the universe: enough to reverse the expansion, not enough, or just right. The designation that scientists had given to the measure determining the fate of the universe was, aptly, the final letter in the Greek alphabet, omega. If the universe contained half the mass necessary to halt the expansion, then you would say omega equaled 0.5, or if it contained three-quarters of the necessary mass, you would say omega equaled 0.75. If the universe contained more than enough mass to halt the expansion, then omega equaled more than 1—1.5 times, maybe, or 2 times, or 100 times. And if the universe contained just the right amount—precisely the critical density to stop the universe from expanding but keep it from collapsing back on itself—then omega equaled 1. Astronomers would even be able to measure omega, if they had a standard candle that they could trace far enough across the universe. But you might not need observations to know omega, Dicke argued. Theory alone might be enough.